Our current project concerns the function of post-translational modifications of microtubule proteins: tyrosinolation of tubulin, and phosphorylation of MAP-2.MAP-2 (MW 270,000) is the principal protein in vertebrate brain (where it is largely confined to dendrites) that coassembles with, and promotes assembly of, tubulin in vitro. MAP-2 as isolated contains about 10 mol of phosphate (A subset). An additional 10 (B subset) can be added by cAMP kinase, a large proportion of which in brain is strongly bound to MAP-2. Two results suggest that A and B phosphates occupy different sites. 1) B phosphates are enriched in the 32-kDa binding domain, whereas A phosphates (those that turn over in vivo) are confined to the 240-kDa projection domain. 2) A phosphates are relatively resistant to all types of protein phosphatase. The practical goal of finding a phosphase with specificity for A sites has been hindered by limited amounts of this labeled substrate. Meantime, using B site labeled MAP-2 as substrate, a brain enzyme has been purified which has relatively high activity with this substrate compared to others (the same is true for brain calcineurin). B site phosphates have been shown to affect microtubule assembly. We wish now to determine the effect of A and B site phosphates on microtubule interaction with neurofilaments and other organelles